Intermittent misfire

[clive]

G80 with SR1 magneto. A while back the little washer under the points broke up and I was relayed home. Replaced the capacitor before working out the missing insulating washer was the problem.
Fast forward not much use made of bike since as there was a new toy in the garage. Some lower speed use but not a huge mileage.
Yesterday out for a 100 mile run. Started nicely run to the meeting point included a mile or so at 65 all good. Due to poor run leadership (mine!) quite a bit of time spent in traffic. Eventually we got to the lanes. Bike started misfiring on load sometimes, ie going up hills, but not on others. Changed plug and cleaned carb bowl below main jet. No improvement. Return journey up A3 at speed. Filled tank with fresh E5 97 petrol. Pulled away same problem. Tried putting on reserve tap in case main tap blocked. No improvement.
So if I carried on driving the misfire tended to clear itself suggesting it was a fuel supply issue. On the A3 on the way back once the misfire stopped i was able to keep a steady 55 for about 15 miles. No problem through heavy traffic once i left the A3.

Advice on what to look for please. My thinking was to start by checking the float and filter atop the monobloc carb. And recheck nothing in the small bowl under the main jet.
Then move on to the magneto check points gap possibly change capacitor for a second similar spare i bought at the same time. It is not specifically for a mag but bought from one of those fast disappearing small car spares shops. Finally change plug again this time for a new one.
Is it possible an internal wiring fault in the mag could give these symptoms? Might the cause be water in the tank, the bike stood a while unused and the garage tends to be damp, the refill was with a 3/4 empty tank. Your thoughts please.

[Groily]

It's always a tricky one when these intermittent hassles arise.
As we all know, all spark problems are fuel and vice versa!
Certainly worth checking flow from both taps, and then cleaning the bits on top of the float bowl, etc, the jet holder below again, plus blowing through the needle and main jets, in case starvation sets in when the throttle is used in earnest. But I have a sense that that probably isn't your problem given what you have done already.

Assuming we can rule out a leaky head joint (which could produce a similar effect under loads), I reckon I'd try swapping the condenser for the spare as a next step, as it's easy and 'no cost'. If the thing had been in stock a long time, it might have deteriorated, as it's what happens to the chemical paper types over time. If you have a meter that measures capacitance, it ought probably to show about 220 nF or 0.22 micro-farad - but that doesn't tell you it isn't leaky, it needs an insulation tester for that. (No guarantee of course that another one from the same batch will be better - one hears tell of whole boxes of new ones being past their best simply owing to the passage of time.)

It's possible there are other 'things', of course, but if the HT lead is good and hasn't part-melted against something hot, if the points aren't shorting any more, and if the plug's a good one, if there are adequate valve clearances, etc etc, nothing to lose by revisiting the last place you played, which was at the condenser.

But, as ever, I'd do one thing at a time or you'll never know what made the difference!

[clive]

Thanks Groily. Not sure I can discount leaking head gasket as there is some oil around the head. This may well be worth investigating. I think the plan will be fuel flow/ carburation, magneto points and condenser followed by the head gasket. In that order because it's easiest first! As you say one section at a time and test after each one so it may be a while before I report back on the conclusion. With an intermittent fault you have got to try it, but the fault not immediately showing up is no guarantee its not still there!!!

[Pharisee]

I had an annoying, intermittent misfire on my '71 Bonneville a while ago. I did all the things you've done with no real success. I did eventually find the cause, however. When my frame had been built at Meriden, the guy who welded it had managed to stick his welding wire to the frame. He'd apparently cut it off with a pair of wire cutters but left a little piece, about 3/32" long, sticking up. It was right where the left-hand HT lead passed and it had eventually worn through the insulation, causing an intermittent short. I ground it off, replaced the HT lead... all good!!
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[clive]

Nice find John! Well i looked in the filter.

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Took the bike out on my test track (otherwise known as the A40 before it turns into the M40) It was pulling an indicated 75 and only bogged down when we reached an incline and i gave it a bit more stick. (Was it really 75 well I fixed and calibrated the speedo myself but i was passing all the other traffic on the derestricted section.) That will do me until i get further problems. 60 mph home without a stutter. When tested the taps pour petrol, should have caught it as it is so expensive! Whilst doing it though i did notice one of the pipes was probably not ethonol resistant ( had a similar one go porous on another bike) so that was replaced. Might it explain the muck in the filter?

[Andy G]

If you're getting that much muck at the carb filter...how is it getting past the in-tank filters of the fuel taps? I'd be taking the taps out to check

[clive]

If you're getting that much muck at the carb filter...how is it getting past the in-tank filters of the fuel taps? I'd be taking the taps out to check

1. Because I don't have any in-tank filters on the EWARTS type taps I am using.
2. Because I think it was from the deterioration of the older pipe (not ethanol resistant) I had connected from the main petrol tap to the carb.

Good point though and once I have used some of the fuel in the recently refilled tank I will take them off and check what comes out of the tank. I do not think it has any sealant inside but memory can be deceptive as its moved around between bikes for the last 30 years! I will be checking all my other bikes for this older type pipe which had a nice material cover and replacing it with fuel pipe rated for use with ethanol petrol. The other bike which had the pipe go was being used mostly on E5 I then filled with E10 in a hurry and 24 hours later the pipe went porous gently wetting the length of the pipe.

[Trefor]

I had similar symptoms (intermittent miss fire over 50mph) after having the magneto rebuilt. I checked everything on the bike to no avail and returned the magneto. The problem was solved when they replaced the points.

[clive]

Yes I have to take the bike out for a longer ride after the last work. cleaning and checking points and possibly replacing the capacitor is my next task if it is not fixed. Trying to work on one area at I time so I actually know what was wrong.

[CharlesDP]

I had one of these once on my 18s, and all it turned out to be was that the breather hole in the fuel cap had blocked over.